Method for validating a downhole connate water sample

Abstract

A downhole connate water sample drawn from the formation surrounding a well is validated when mud filtrate concentration is acceptably low. A preferred method includes drilling the well with a water-based drilling fluid, or more generally a water-based mud (WBM), containing a water-soluble dye. The dye acts as a tracer to distinguish connate water from WBM filtrate in a downhole sample of formation fluid contaminated by mud filtrate from the water-based mud. Preferably, an optical analyzer in a sampling tool measures light transmitted through the downhole sample to produce optical density data indicative of dye concentration. Preferably, optical density is measured at a first wavelength to obtain a first optical density, and at a second wavelength, close in wavelength to the first wavelength, to obtain a second optical density. First and second optical density data are transmitted to the surface. At the surface, in a data processor, the second optical density is subtracted from the first optical density to produce a third optical density that is substantially free of scattering error. The data processor validates each sample that has an acceptably low third optical density. The invention also provides a method of determining when to collect a sample of downhole fluid drawn over a period of time from a formation surrounding a well.

What is claimed is:  
     
       1. A method for validating a downhole connate water sample drawn from formation surrounding a well, comprising: 
       drilling the well with a water-based mud containing a water-soluble dye;  
       obtaining a sample of formation fluid downhole;  
       measuring optical density of the sample downhole; and  
       validating the sample if sample optical density is acceptably low.  
     
     
       2. A method according to  claim 1 , further repeating said act of obtaining a sample of formation fluid downhole and said act of measuring optical density of the sample downhole to obtain optical density from each of a series of samples. 
     
     
       3. A method according to  claim 1 , wherein said water-soluble dye is a blue dye. 
     
     
       4. A method according to  claim 1 , wherein said water-soluble dye is a dye selected from a group of dyes, the group consisting of Acid Blue #1 (EMI-600) and Acid Blue 9, alphazurine FG. 
     
     
       5. A method according to  claim 1 , wherein said water-soluble dye is a dye that is active in the ultraviolet region of the spectrum. 
     
     
       6. A method according to  claim 1 , wherein said water-soluble dye is a fluorescent dye. 
     
     
       7. A method according to  claim 1 , wherein said water-soluble dye is added to said water-based mud to produce a concentration within the range 0.2-2.0 kg/m 3  (200-2000 mg/L). 
     
     
       8. A method according to  claim 1 , wherein measuring optical density includes measuring optical density at a first wavelength to obtain a first optical density, measuring optical density at a second wavelength to obtain a second optical density, and subtracting said second optical density from said first optical density. 
     
     
       9. A method according to  claim 8 , wherein said first wavelength and said second wavelength are close in wavelength. 
     
     
       10. A method according to  claim 1 , further comprising: 
       determining scattering from a series of optical density values; and  
       validating a sample if the scattering is acceptably low.  
     
     
       11. A method according to  claim 1 , further comprising: 
       calculating from a series of optical density values an asymptotic value indicative of water-based mud filtrate fraction; and  
       validating a sample if the asymptotic value is stable.  
     
     
       12. A method for validating a downhole connate water sample drawn from formation surrounding a well, comprising: 
       drilling the well with a water-based mud;  
       obtaining a sample of formulation fluid downhole;  
       measuring at least one characteristic of downhole fluid indicative of water-based mud filtrate contamination levels in the sample; and;  
       validating the sample if the at least one measured characteristic is acceptably low.  
     
     
       13. A method according to  claim 11 , wherein said at least one measured characteristic is optical density. 
     
     
       14. A method according to  claim 11 , wherein said at least one measured characteristic is fluorescence emission, ion concentration, or relative ion concentration. 
     
     
       15. A method according to  claim 11 , wherein said water-based mud contains a predetermined salt concentration, and wherein said at least one measured characteristic is conductivity or resistivity. 
     
     
       16. A method for determining when to collect a sample of downhole fluid drawn from a formation surrounding a well, comprising: 
       measuring at least one characteristic of downhole fluid indicative of water-based mud filtrate contamination levels in downhole fluid drawn from a formation surrounding the well over a period of time; and  
       using said measurements to determine when to collect a sample of said downhole fluid.  
     
     
       17. A method according to  claim 16 , wherein said characteristic is optical density, fluorescence emission, conductivity, resistivity, ion concentration, or relative ion concentration. 
     
     
       18. A method according to  claim 16 , wherein said water-based mud filtrate contains a water-soluble dye.